The zeta subunit of the T cell antigen receptor is a limiting component in receptor assembly and is required for the targeting of the T cell antigen receptor to the cell surface. Zeta is differentially regulated relative to the other T cell receptor components as witnessed by its expression in natural killer cells. It is also clear that this subunit undergoes alternative splicing to yield another product that has been characterized in murine cells and termed eta. We have been studying the gene encoding the human zeta subunit. Our gene; the determination of sites of transcription initiation; the characterization of this gene as a genetic marker; and the characterization of the human eta equivalent. As an extension of these findings we have also ben studying the relative expression of zeta and eta on a message and protein level during thymic development.